Also known by QB, Kyubey is one of the main characters from Mahou Shoujo Madoka Magica. He is a "Messenger of Magic" (魔法の使者 mahou no shisha) who grants the wishes of young girls, but in exchange contracts their services as Puella Magi (魔法少女, mahou shoujo, lit. Magical Girls). Girls who form a contract with him are tasked to defeat Witches -- bizarre entities that feed upon the hopes and dreams of normal people.
Puella Magi Wiki: http://wiki.puella-magi.net/Kyuubey